[simpits-tech] GPS craziness and "desired track"

[Matt Bailey]

On Sat, 5 Apr 2014 20:13:02 +0200
Jeroen Huijben <jeroen_huijben at hotmail.com> wrote:

> Your method of finding where you should be sounds right.
> But if you want to fly straight to this point you will have to fly
> perpendicular to the desired track and if you do this until you reach
> it you will overshoot and end up snaking along the desired track. So
> you should add something to start the turn before reaching the
> desired track and make a smooth transition.

This "intermediate" point isn't something I would navigate to, its sole
purpose would be to find a bearing solution from this point to my next
waypoint, so that I would get the correct desired-track value for that
point in the Great Circle route.

> You could add a warning to start a standard rate turn when you are
> one turn radius away from the desired track. That should place you on
> the track while flying along it. Then you can switch to a course
> deviation indicator to show how far away from the track you are.

I am actually tied to the operation of the old 1990s Trimble GPS units,
as I am basically re-implementing the Trimbe "OS" in Python (so that
it can be interfaced to X-Plane, or even FSX/FlightGear/et al).
The CDI is always available. I'm pretty sure the only "help" it's
suppose to give the pilot is a fly-to heading, which I already have
written into my software. I do not yet have it set up for course
corrections, but I'm thinking I will simply make it a percentage of the
course error.

One thing I'm not sure about is if the CDI/heading corrections are
based on bearing, or cross-track error. I'm assuming it's bearing, as
this is how a VOR works. Otherwise, you could be almost past the
waypoint, but have only a small CDI deflection because your actual
cross-track distance might be fairly small (a couple miles or so, for
instance). But then maybe a real GPS works this way, I'm not sure.

I'm tempted to yank the Trimble antenna off the Sabreliner, rig up a
portable power unit for the GPS, and take it up in a real plane so I
can see how it behaves in the air. :)

> You could even seamlessly integrate each part and present only a CDI
> to the pilot. To reach the desired track you will first fly a great
> circle perpendicular to it. Then you fly along a small circle that
> touches the intercept track and the desired track. Finally you fly
> along the great circle that is the desired track. In each case the
> CDI would show the distance from the active great or small circle.

By "small circle" are you referring to something other than a Great
Circle, or do you mean just a shorter Great Circle? :)

Matt